The pressing need for advanced battery technologies constitutes the driving force in developing new electrode formulations to replace conventional intercalation compounds and carbonaceous materials in current lithium-ion batteries. Electrochemically active metals and metalloids that can form intermetallic alloys with lithium, such as silicon, germanium, and tin, as well as transition metal oxides that can react with lithium ions reversibly via conversion reactions, such as tin dioxide, iron oxide, and manganese dioxide, have great potential to radically boost the energy density of lithium-ion batteries. Nevertheless, despite their promise as electrode materials, remarkable volumetric expansion/contraction may occur in these materials during charge/discharge cycling as a result of the lithiation/de-lithiation processes. These large volumetric changes often result in pulverization of the electrode materials. Once fragmented in this manner, side reactions may then occur at the freshly formed electrode/electrolyte interfaces, and the electrode fragments may become isolated by the newly formed side products and lose electrical contact. These unwanted side reactions gradually deplete the available electrolyte, and severely hinder the rate capability and deep cycling ability of the electrodes. By reducing particle size and dispersing the electrode materials into high content conducting additives and polymer binders, such issues with pulverization can be partially addressed at the expense of tapped density, overall capacity, and energy density of the resultant devices.
Incorporating graphene sheets into the high-capacity active materials offers an alternative solution to suppress the detrimental effects of volumetric variation, although this technology is not admitted as prior art with respect to aspects of the present invention by its discussion in this Background Section. A graphene composite electrode 100 employing graphene platelets formed from the exfoliation or the separation of graphite flakes is shown in FIG. 1. Here, graphene platelets 110 are distributed among electrochemically active nanoparticles 120 in a polymer binder 130. In such a system, the graphene platelets 110 help to accommodate lithium ion insertion/extraction stress during cycling and also supply the necessary electrical conductivity. That said, the preparation of uniform graphene composites remains a major challenge in designing desirable electrode systems, and the reinforcing effect from graphene platelets in such composites is far below what has been envisioned. Such electrode systems, for example, suffer from a strong tendency towards phase segregation, and graphene quality and morphology are typically difficult to control. In addition, the electrochemically active nanoparticles are prone to detachment from the graphene platelets and to re-agglomeration during cycling because of non-intimate contact at the graphene/active material interfaces.
Graphene oxide (GO), the oxidized form of graphene that may be obtained through treatment of graphite powder with oxidizing agents, has also been investigated as an electrode additive because of its excellent surface functionality and reactivity. Researchers have synthesized metal and oxide electrochemically active nanoparticles partially encapsulated by GO sheets by, for example, generating opposite surface charges and electrostatic attraction at the interface between the GO and nanoparticles, although, again, this technology is not admitted as prior art by its mention in this Background Section. In this manner, improved electrochemical performance has been demonstrated after reducing GO to restore the aromatic carbon networks. However, the GO-based structures remain highly defective and resistive even after reduction, which is not optimal for high-performance energy storage (e.g., battery) applications.
For the foregoing reasons, there is a need for alternative electrode technologies for use in high-performance energy storage devices such as batteries and supercapacitors that do not suffer from the several disadvantages described above.